Over the past three decades, polymer chemists and soil scientists have developed controlled-release pesticides for agricultural use. The two primary goals of controlled-release pesticides are (1) to increase efficacy of the pesticide and (2) to reduce negative environmental consequences of pesticide application. Some prior art controlled-release pesticides have been encapsulated in starch. These prior art starch-encapsulated controlled-release pesticide products are typically formed by mixing starch and pesticides and forming balls of the mixture. These starch-encapsulated pesticides provide rate-limited release of the pesticide particles from the starch, which release is governed largely by diffusion. Specifically, when starch-encapsulated pesticides are applied to the soil, they imbibe water and swell such that the pesticide particles diffuse out of the starch matrix into the soil surrounding a plant, root, seed, or seedling.
In 1976, polymer chemists developed a class of materials referred to as superabsorbent polymers (SAPs) (see, e.g., U.S. Pat. Nos. 3,935,099; 3,981,100; 3,985,616; and 3,997,484, all issued in 1976). SAPs are materials that imbibe or absorb at least 10 times their own weight in aqueous fluid and that retain the imbibed or absorbed aqueous fluid under moderate pressure. The imbibed or absorbed aqueous fluid is taken into the molecular structure of the SAP rather then being contained in pores from which the fluid could be eliminated by squeezing. Some SAPs can absorb up to 1,000 times their weight in aqueous fluid.
One type of SAPs, called “totally synthetic copolymers,” is made by copolymerizing acrylic acid and acrylamide in the presence of a coupling agent. Almost all totally synthetic copolymer SAPs are used in baby diapers, adult diapers, catamenials, hospital bed pads, cable coating, and the like. Today the worldwide market for totally synthetic copolymer SAPs is estimated to be about 2 billion pounds per year.
Another type of SAPs, called starch graft copolymers, use a natural polymer, such as a starch, to form an SAP product including a starch graft copolymer. Films of starch graft copolymer SAP are typically formed by drying the starch graft copolymer composition on a tray or heating the composition on a drum dryer. The resulting films can then be ground or milled into flakes or powders. Films of starch graft copolymer SAP may also be made by diluting a viscous mixture of alkali starch graft copolymer with a water-miscible organic solvent such as alcohol or acetone to precipitate an alkali starch graft copolymer. The precipitated alkali starch graft copolymer is then isolated in a fine, powdery form by filtration and additional drying. Starch graft copolymer SAP products that absorb large quantities of aqueous fluids are typically marketed as absorbent soft goods that increase the water-holding capacity of soil and that form a coating on fibers, clay, paper, and the like.
The aqueous fluid absorption capabilities of SAPs have long made them desirable to agricultural companies. However, testing of the totally synthetic copolymer SAPs and the film or powdery starch graft copolymer SAPs showed poor agricultural performance, largely due to the particle size of the SAP products (small, fine particles measuring about 80 mesh in size). One inherent limitation of finer-mesh particles is that they cannot be used in typical granule applicators, which require particle sizes of at least 25 mesh. Further, the fine powders and/or films are often carried away by any wind present during application of the SAP product onto a field or a growing substrate.
While pesticide particles mixed with starch have been manufactured for many years, no one has successfully entrapped pesticides in a starch-based SAP product appropriate for use in large-scale agricultural applications. The inventors of the present invention recognized a need in the agricultural industry for a method of forming a starch-based SAP product including a bioactive, growth-promoting additive whose application to plants, roots, seedling, or seeds, or to a growing substrate in proximity to plants, roots, seedling, or seeds, promotes growth of the plants, roots, seedling, or seeds.